Rice water weevil (RWW) is the most important insect pest of California rice. The industry has efficiently switched from the preventative, preplant mode of RWW control to the post-flood timing required with these products — Dimilin^® 2L and Warrior^®.  Researchers sought to optimize use patterns for these new insecticidal controls to maximize efficacy, reduce costs and refine management activities.  Research conducted in 2002 also progressed on important aspects of rice water weevil biology, as well as some armyworms and “peck” rice and its possible causes.

Light RWW Flight

The timing of RWW adult flight in the spring has been monitored for nearly 40 years at the Rice Experiment Station.  Monitoring weevil flight is important to determine the levels and intervals of peak flight periods and to compare RWW trends over time.  The switch to the use of post-flood insecticides has placed even greater importance on understanding RWW flight timing.

The RWW flight in spring 2002 was the lightest in recent years.  Only 655 RWW were captured last year, compared to more than 8,000 in 2001, which was the heaviest flight in five years.  Reasons for this fluctuation are unknown, but the flight intensity had been on an upward path until 2002. 

The flight was earlier than normal, with 90 percent completed by May 11. Peak flight time included April 12, April 22-23, May 3 and May 25.  Normally the flights last for several days, but in 2002 they were concentrated on single days.  Heavy RWW populations were reported in some grower fields during 2002.

Chemical Controls

Studies continued last year in ring plots, large field plots and in grower fields to evaluate experimental materials compared to registered standards for RWW control and to refine use of existing products.

In ring plots, F0570 or zeta-cypermethrin (Mustang^®) provided excellent RWW control and also appears to be a viable option for combining with Dimilin^® in the same manner that Warrior^® is combined.  This product is in the same chemical class as Warrior and is on track for registration.

With the consolidation of the agrichemical industry and the challenges of registering a toxicant for the aquatic rice environment, new materials being put forth are limited.  Only three other experimental materials were tested.  XDE 225, in its first year of testing,  provided excellent RWW control through a post-flood application. Cruiser^® seed treatment did not perform well, a surprising finding because the same insecticide tested as a soil treatment four years ago worked very well. This treatment needs to be re-evaluated to rule out the possibility of an improper technique.  Icon^® was the final experimental material tested in 2002 and results as a soil treatment were excellent.

Warrior^® applied preplant proved an effective RWW control in 2002, as it had done the previous year.  The effect of time between application to the bare soil and flooding was examined in ring plots.  At six and two days before flooding, immediately before flooding, and immediately before seeding, RWW control was described as universally excellent.

The effect of an adjuvant, Mor-Act, on RWW was examined alone and in combination with Dimilin.  The material itself had no activity on RWW, but it did increase the performance of Dimilin slightly. 

Larger scale trials were conducted with zeta-cypermethrin, Dimilin, Warrior and a combination of Dimilin and Warrior.  All treatments proved effective and it was difficult to separate the treatments.

Warrior preplant reduced adult feeding to only two percent, compared to 76 percent in untreated plots.  The mode of action of this preplant application has been unclear, but adult RWW are clearly being killed.  Researchers suggest that either a concentration of the active ingredient accumulates in the water from the soil or, conversely, the adults may forage in the soil/mud and become exposed to the toxicant.  Yield data show the highest grain yields with zeta-cypermethrin, Dimilin 2L and Warrior pre-plant treatments.

Warrior is registered on California rice for post-emergence application.  In 2002, however, research was also conducted in two grower fields with Warrior preplant.  RWW infestations were described as low to moderate.  At one location, plots treated with preplant Warrior had slightly more RWW larvae than plots treated with standard timing.  Based on positive results from ring plots in 2002 and the acceptable (although not definitive) data from the grower field tests, the Warrior preplant application appears to be viable. 

In terms of grain yield, Warrior preplant was highest yielding at one site, while Warrior postflood was highest yielding at the other site. Rice grain yields ranged from about 4,760 pounds/acre to 7,650 pounds/acre — comparable to previous years.

RWW Biology

At present no California rice varieties contain significant RWW resistance.  Nine California varieties were evaluated and compared for susceptibility and response to RWW.  The goal was to determine variations in susceptibility to RWW infestation and to quantify effects on yield.  Percentage scarred plants ranged from 3.75 percent to 12.5 percent. The highest percentage of adult scarring occurred on M-205, M-401 and M-202; the lowest on Calmati 201.  Yields of L-205 and Calhikari 201 were not reduced by RWW larvae, although the infestation would be classified as moderate.  Overall, the medium grains appear to be fairly tolerant of weevil feeding but can support higher larval populations.  The specialty rice varieties suffered the greatest yield loss.

The applicability of a floating barrier trap for monitoring adult RWW populations in grower fields was further studied at nine locations.  Overall, RWW were captured at eight of nine sites, with a peak of 25 adults per trap per day in one field.  A high percentage of the adults was captured by the first leaf stage (58 percent) and by the second leaf stage (86 percent).  This bodes well for the usefulness of the trap, as this period corresponds to when management decisions must be made.

The relationship between the “catch” in these floating barrier traps and larval populations was examined.  Although there was considerable variation in the data, the results showed a linear relationship between adult captures and the resulting larval numbers and for every adult captured in a trap about 0.6 larva resulted.  Similar studies showed about one larva per every trapped adult.  Studies to determine length of time needed to prevent yield loss from RWW were hampered by a seed midge infestation.

Length of Rice Water Weevil flight period (completion of 90% of the seasonal flight) at the Rice Experiment Station; 1990-2002

Other Arthropod Pests

“Peck” rice, primarily from the rice stink bug, is a problem that impacts Southern rice production.  This insect does not occur in California.  However, related insects that can cause the same type of damage are common here.  Virtually any insect with sucking mouthparts could, in theory, penetrate, feed on and damage developing rice kernels.  Other environmental and plant disease factors can cause this condition as well.  The grain of pecky rice is discolored and deformed, negatively affecting grain quality and grower returns.  Therefore, it is important to determine the exact cause.

An area that had this problem in 2001 was visited in July 2002 to search for possible causes. A preliminary survey done in 2002 in an area that has experienced peck rice found five different insect species that could be involved, although no direct causal relationship has been established.  Follow-up studies are planned.